Of methods of separating suspended substances present in raw water, a method comprising adding a flocculant to the raw water, allowing the raw water to flow as an upward flow in a separation tank to aggregate pollutants and settling and separating them as coarse particles (flocs), is broadly used.
The treatment velocity in the conventional aggregation precipitation method of the upward flow type (the term “treatment velocity” means herein a flow velocity found by dividing the water amount of the water to be treated by the sectional area of a tank) is restrained by the settling velocity of flocs and cannot be faster than the settling velocity of the flocs. Further, even if the treatment velocity is lower than the settling velocity of the flocs, when the flow velocity is not uniform, the settling of flocs is hindered. Particularly, the upward flow velocity in the neighborhood of the influent section of the raw water is higher than the settling velocity of the flocs, and not only hinders the settling of flocs but also invites the fragmentation of flocs. This can be said independently of whether or not the formation of a blanket layer, in other words, a retention layer of coarse flocs is caused in the course of treatment.
In the aggregation precipitation of the upward flow type accompanying the formation of a blanket layer of flocs, fine flocks and an unaggregated turbidity component are brought into contact with flocs forming the blanket layer and trapped in passing through the blanket layer and as a result, the elimination of turbidity is accelerated, and simultaneously the settling properties of flocs are improved. However, when the blanket layer is formed to improve the settling properties, with increased treatment velocities, the formation of the blanket layer is hindered to cause an overflow of flocs.
On the other hand, as the treatment system which makes the treatment velocity higher than that of the conventional aggregation precipitation treatment, there is a system of installing a concentration tank below a separation tank and connecting the separation tank with the concentration tank by a floc transfer pipe. The flocs formed in the separation tank settle in the flock transfer pipe by the descending flow caused by the settling of flocks as such, and are transferred to the concentration tank, then concentrated and discharged. The separated water to be formed by the concentration process is affected by the above-described descending flow of flocs and returned as a flow to the separation tank.
However, according to this system, the transfer of flocs and water from the separation tank to the concentration tank depends on the descending flow caused by the settling of the flocs as such, and accordingly it is impossible to arbitrarily control the amount of water to be transferred from the separation tank to the concentration tank, and the treatment velocity must depend on the settling velocity of flocs. Furthermore, the function of the concentration tank is only to gravitationally concentrate the influent flocs flown from the separation tank and has no function of positively effecting solid-liquid separation to obtain clear separated water as the separation tank.
As a problem different from the above described problem in the aggregation precipitation method of the upward flow type, the deterioration of the turbidity elimination performance in start-up early stages of the apparatus can be mentioned. This deterioration is a very serious problem in the treatment system accompanying the formation of a blanket layer. Once the blanket layer is formed, fine flocs are brought into contact with large flocs forming the blanket layer, integrated with the flocs, and removed. But in order to form such a blanket layer, it is necessary for fine flocs to repeatedly associate with one another. Thus, on start-up of the apparatus, the treatment is started in a state in the absence of in the blanket layer, and accordingly the property of the treated water is deteriorated until the blanket layer is formed.
The present invention solves the above described problems of the conventional technique and has an object to provide a compact aggregation precipitation method and an apparatus therefor which prevent the outflow of flocs even in the circumstances that the treatment velocity is higher than the settling velocity of flocs and, simultaneously, do not deteriorate the turbidity elimination performance even in start-up early stages of the apparatus.